The present invention relates to a steel product for machine structural use having excellent machinability, and to a structural steel part for machinery manufactured from the steel product. More particularly, the invention relates to a steel product for machine structural use having excellent machinability, particularly bringing about excellent xe2x80x9cdrill lifexe2x80x9d and exhibiting excellent xe2x80x9cchip disposabilityxe2x80x9d in the course of drilling, as well as to a structural steel part for machinery manufactured from the steel product.
In the manufacture of various structural steel parts for machinery, a steel product is roughly formed into a predetermined shape through hot working, such as hot forging, and is then finished into a desired shape through machining. The thus-finished parts may be used in a non-heat-treated state or after being subjected to heat treatment, such as normalizing, normalizing-tempering, or quenching-tempering. Alternatively, a steel product may undergo heat treatment after being subjected to hot working, and may then be finished to a desired shape through machining. Before use, some parts may undergo surface hardening, such as carburizing, nitriding, or induction hardening, serving as a final treatment.
Steels having excellent machinability are classified, according to a machinability-enhancing element(s) added, into S (sulfur) type, Pb (lead) type, Sxe2x80x94Pb type, Ca type, Sxe2x80x94Pbxe2x80x94Ca type, Ti type, and graphite type. In many cases, among these free-cutting steels, a resulfurized free-cutting steel, a leaded free-cutting steel, and a calcium deoxidized steel are employed in structural applications for machinery that requires hard structural steel parts serving as final products. Since machinability of steel deteriorates with hardness, machinability is improved through addition of a large amount of one or more machinability-enhancing element(s), such as Pb, S, or Ca.
However, addition of a large amount of Pb, S, or Ca may cause occurrence of a defect in structural steel parts for machinery, or final products. For example, addition of a large amount of Pb, S, or Ca causes coarsening of inclusions; hence, surface hardening, such as induction hardening or carburizing, may involve occurrence of quenching cracks, which may remain in final products.
Addition of a large amount of Pb, S, or Ca to steel inevitably involves impairment in toughness. Thus, the above-mentioned conventionally popular free-cutting steels may be employed as steel stock without any problem in manufacture of structural steel parts for machinery requiring only moderate toughness, such as crank-shafts, connecting rods, and printer shafts, but may encounter difficulty in obtaining a desired high toughness in manufacture of structural steel parts for machinery requiring high toughness, such as wheel hubs, spindles, knuckle arms, and torque arms. For example, in manufacture of high-hardness structural steel parts for machinery requiring a Vickers hardness of not less than 160, the above-mentioned free-cutting steels to be employed contain a large amount of S so as to enhance machinability, and a large amount of Pb so as to enhance chip disposability. As a result, anisotropy of toughness increases, and toughness itself is impaired significantly.
As one measure to cope with this problem, PCT Pub. No. WO98/23784 discloses a free-cutting steel product for machine structural use, which contains Ti in an amount of 0.04 to 1.0% by mass in the form of a finely dispersed Ti carbosulfide to thereby exhibit excellent machinability. The free-cutting steel product proposed in this publication can suppress occurrence of a defect in final products, which would otherwise result from coarsening of inclusions, and can impart favorably balanced hardness and toughness to structural steel parts for machinery. However, industrial demands for enhancement of machinability are growing further. Recently, a further increase in cutting speed has been sought in order to further reduce cutting cycle times in automated production lines. In order to meet these demands, there has been demand for steel products for machine structural use surpassing the proposed steel product in machinability.
Japanese Patent Application Laid-Open (kokai) No. 49067/1997 discloses a new technique for enhancement of machinability; specifically, xe2x80x9csteel for plastic moldxe2x80x9d having an increased Si content. However, when employed as steel stock in manufacture of structural steel parts for machinery, the proposed xe2x80x9csteel for plastic moldxe2x80x9d fails to provide stable chip disposability required in cutting of parts in an automated mass production line, as in cutting of automobile parts, such as connecting rods and gears. Since molds are machined individually while in an open state, chip disposability does not raise any problem in machining thereof. Accordingly, the invention of the proposed xe2x80x9csteel for plastic moldxe2x80x9d does not take chip disposability into consideration.
An object of the present invention is to provide a steel product for machine structural use having excellent machinability; specifically, bringing about excellent xe2x80x9cdrill lifexe2x80x9d and exhibiting excellent xe2x80x9cchip disposabilityxe2x80x9d in the course of drilling therein a so-called xe2x80x9cdeep holexe2x80x9d having a (hole depth)/(hole diameter) ratio of not less than 5 by use of a drill made of a conventional Co-containing high-speed steel (a so-called xe2x80x9chigh-speed steel drillxe2x80x9d), as well as to provide a structural steel part for machinery manufactured from the steel product. Herein, a steel product for machine structural use and a structural steel part for machinery of the present invention have a target Vickers hardness (hereinafter called Hv hardness) of 160 to 350 and bring about a xe2x80x9cdrill lifexe2x80x9d of not less than 150 drilled holes. Specific examples of structural steel parts for machinery that must have these characteristics include crank-shafts, connecting rods, and printer shafts.
Another object of the present invention is to provide a steel product for machine structural use exhibiting an absorbed energy at room temperature (UERT) of not less than 40J as measured in an impact test conducted by use of a No. 3 test piece for a Charpy impact test specified in JIS Z 2202 as well as having an Hv hardness of 160 to 350 mentioned above and machinability mentioned above in terms of xe2x80x9cdrill lifexe2x80x9d and xe2x80x9cchip disposability,xe2x80x9d as well as to provide a structural steel part for machinery manufactured from the steel product. Examples of structural steel parts for machinery that must have these characteristics include wheel hubs, spindles, knuckle arms, and torque arms.
Notably, an Hv hardness of 160 to 350 corresponds to a tensile strength of about 520 to 1100 MPa.
The gist of the present invention is as follows:
A steel product for machine structural use having a chemical composition comprising, in mass percent, C: 0.05% to 0.55%; Si: 0.50% to 2.5%; Mn: 0.01% to 2.00%; P: not greater than 0.035%; S: 0.005% to 0.2%; Cu: 0% to 1.5%; Ni: 0% to 2.0%; Cr: 0% to 2.0%; Mo: 0% to 1.5%; V: 0% to 0.50%; Nb: 0% to 0.1%; Ti: 0% to less than 0.04%; B: 0% to 0.01%; Al: not greater than 0.04%; N: not greater than 0.015%; Bi: 0% to 0.10%; Ca: 0% to 0.05%; Pb: 0% to 0.12%; Te: 0% to 0.05%; Nd: 0% to 0.05%; Se: 0% to 0.5%; value of fn1 represented by equation (1) below: not less than 0; value of fn2 represented by equation (2) below: not less than 3.0; and balance: Fe and incidental impurities; an area percentage of a ferrite phase in a microstructure being 10% to 80%; and Hv hardness being 160 to 350;
fn1=xe2x88x9223C+Si(5xe2x88x922Si)xe2x88x924Mn+104Sxe2x88x923Crxe2x88x929V+10xe2x80x83xe2x80x83(1)
fn2=3.2C+0.8Mn+5.2S+0.5Crxe2x88x92120N+2.6Pb+4.1Bixe2x88x920.0xcex12+0.13xcex1xe2x80x83xe2x80x83(2)
where an element symbol appearing in equation (1) or (2) represents the content in mass percent of the corresponding element, and a represents the area percentage in % of the ferrite phase in the microstructure.
Preferably, in order to obtain sufficient toughness, in the above-mentioned chemical composition of the steel product for machine structural use, the S content, in mass percent, is 0.005% to 0.080%, and the value of fn3 represented by equation (3) below is not greater than 100.
fn3=100C+11Si+18Mn+32Cr+45Mo+6Vxe2x80x83xe2x80x83(3)
where an element symbol appearing in equation (3) represents the content in mass percent of the corresponding element.
Preferably, in the above-mentioned chemical composition of the steel product for machine structural use, the S content in mass percent is 0.005% to 0.080%; the value of fn3 represented by equation (3) is not greater than 100; and the value of fn4 represented by equation (4) below is not less than 5.0, thereby imparting sufficient toughness to a structural steel part for machinery. In this case, structural steel parts for machinery formed from the steel product through hot forging can be free from occurrence of a defect which would result in rejection thereof as defective articles in nondestructive testing, such as ultrasonic testing or magnetic particle testing. Furthermore, when the structural steel parts for machinery are subjected to surface hardening serving as a final treatment, such as carburizing or induction hardening, cracking of the structural steel parts can be prevented.
fn4=n1/n2xe2x80x83xe2x80x83(4)
where n1 represents the number of inclusions having a maximum diameter of 0.5 xcexcm to 3 xcexcm, and n2 represents the number of inclusions having a maximum diameter in excess of 3 xcexcmm as observed in a longitudinal section of the steel product.
Preferably, in order to enable structural steel parts for machinery to bring about longer drill life, in the above-mentioned chemical composition of the steel product for machine structural use, the Mn content in mass percent is 0.15% to 2.00%; the S content in mass percent is in excess of 0.080% and not greater than 0.2%; and the value of fn1 represented by equation (1) is not less than 7.5.
Drilling conditions are as mentioned previously; specifically, a so-called xe2x80x9cdeep holexe2x80x9d having a (hole depth)/(hole diameter) ratio of not less than 5 is drilled by use of a conventional Co-containing high-speed steel drill. The above-mentioned xe2x80x9cholexe2x80x9d may be a so-called xe2x80x9cblind hole,xe2x80x9d which does not extend through the object of drilling along a drilling direction, or may be a xe2x80x9cthrough-hole,xe2x80x9d which extends through the object of drilling.
When a single hole is drilled, chips other than a chip which is ejected from a drill tip immediately after start of drilling break in various shapes. Fn2 represented by equation (2) serves as the xe2x80x9cindex of chip disposabilityxe2x80x9d indicative of xe2x80x9cchip disposabilityxe2x80x9d. FIG. 1 shows the relationship between the value of fn2 and the state of breakage of chips. Values of fn2 equal to or less than 0 are all defined as xe2x80x9c0xe2x80x9d.
The area percentage in microstructure is obtained through microscopic observation.
In the present invention, the xe2x80x9clongitudinal sectionxe2x80x9d (hereinafter, called an xe2x80x9cL-sectionxe2x80x9d) of a steel product denotes a section of the steel product taken along a centerline of the same in parallel with a machining direction. The xe2x80x9cmaximum diameterxe2x80x9d of an inclusion denotes a diameter as measured across xe2x80x9cthe widest portion of an inclusion on an L-section.xe2x80x9d